At present, filters employing surface acoustic wave devices have widely been used mainly for mobile phones and the like. In general, such a filter is composed of a resonator in which an IDT (comb-shaped electrode) and a reflector are provided on a piezoelectric substrate. In such a filter, characteristics required for the surface acoustic wave device depend on a frequency band and a system, and thus it is important to use a piezoelectric substrate suited to the frequency band and the system. Here, the frequency band and the pass band are greatly influenced by a surface acoustic wave velocity and an electromechanical coupling coefficient, and their characteristics largely depend on the material and cut angle of the piezoelectric substrate.
In the piezoelectric substrate, if there is no symmetry of crystal about the direction in which the wavefront of a surface acoustic wave propagates, the phenomenon called beam steering occurs in which the wavefront propagation direction (the direction of phase propagation) is different from the direction of energy travel. The angle formed between the wavefront propagation direction and the direction of energy travel is called power flow angle. If the power flow angle is high, an energy loss occurs and thus the filter characteristic may deteriorate. Therefore, using a cut angle at which the power flow angle is zero is considered, but optimal surface acoustic wave velocity and electromechanical coupling coefficient are not necessarily obtained with such a cut angle. For that reason, it is important how the loss is reduced in a state where there is beam steering.
As a filter that achieves this, a filter composed of a resonator as disclosed in Patent Literature 1 is proposed. FIG. 10 shows a top view of the filter 900. In the filter 900, a pair of IDT electrodes 901 and reflectors 902 are formed on a piezoelectric substrate 910. A plurality of electrode fingers 903 of each IDT electrode 901 are arranged so as to be parallel to each other such that the direction in which the electrode fingers 903 extend is orthogonal to an X axis that is the direction in which the wavefront of a surface acoustic wave propagates. In addition, the direction along which these electrode fingers 903 are arranged is the direction along an X′ axis which is tilted relative to the X axis by a power flow angle ζ. Furthermore, the reflectors 902 provided on both sides of the IDTs are arranged along the X′ axis. Moreover, a plurality of conductor strips 904 constituting each reflector 902 are arranged so as to be parallel to each other such that the direction in which the conductor strips 904 extend is orthogonal to the X axis. Thus, intersection areas of the electrode fingers 903 and the reflectors 902 are arranged along the direction of energy travel, and hence the energy can be confined and the loss can be reduced.